Attenuation of solar UV radiation by aerosols during air pollution episodes

Increases in solar ultraviolet radiation reaching the surface due to stratospheric ozone depeletion continues to be a major concern. However, recent studies have shown that the boundary layer air pollution decreases the UV radiation reaching the surface due to particle scattering and absorption. In this study, we examined solar ultraviolet radiation measured during 1994 to 1998 by a Brewer Spectrophotometer at Gaithersburg, MD for effects of particle scattering on UV radiation during air pollution episodes. Both transport from upwind regions and local sources result in severe pollution episodes at Gaithersburg when meteorological conditions are favorable. During air pollution episodes, atmospheric column aerosol loading is high; the mean value of aerosol optical depth measured by a sunphotometer in the UV (340 nm) on all smoggy days during 1994 to 1998 is about 1.02 ± 0.32. Both observations and radiative transfer model calculations of two severe air pollution episodes show that particle scattering by aerosols (increase in optical depth of about 1.0 at 340 nm) reduces Erythemal UV flux by up to 15% and spectral UV flux by 17 to 19% depending on wavelength. To isolate the effect of aerosols on UV radiation from the combined effect of aerosols and column ozone, we analyzed the UV flux at 340 nm. We found that UV flux at 340 nm decreases at a rate of 80 mW/m2/nm per unit aerosol optical depth. While decreased UV flux may reduce the ozone production potential near the surface, it may increase the lifetime of photodegradable carcinogenic pollutants in the air and aquatic systems.